Rails: "currency" is escaped/transformed to "¤cy" without my asking

Rails: "currency" is escaped/transformed to "¤cy" without my asking

I have hit upon a very strange thing in my Rails 3.2.9 app - when I render
this:
render inline: BusinessesController.checkout_page_url
with this method:
def self.checkout_page_url(business, order_number)
url = ''
# ...
url << 'currency=' << conf['currency'] << '&'
# ...
puts url
return url
end
then the resulting url is rendered just fine in the console (puts url):
... currency=EUR& ...
but the rendered string in the browser reads:
... ¤cyEUR& ...
I've tried calling .html_safe on the string before rendering it, but
didn't change a thing. What's going on?